In the early days of racquet sports, the traditional material used to string sports racquets was a natural gut material made from animal fibers. In the sport of tennis, players found that natural gut was an acceptable material because of its good tensile modulus which produces an excellent combination of elasticity and resiliency. The desirable tensile modulus found in natural gut has been attributed, at least in part, to its multiple fiber construction. Natural gut remains one of the most favored strings for use in tennis sports today.
However, natural gut has some drawbacks and these disadvantages have traditionally discouraged its universal use. For example, it is adversely affected by humidity, frays easily and is not considered very durable. Additionally, the available amount of raw material is limited. Furthermore, it is difficult to produce strings with a constant modulus property because of the varying character of the raw material. All these factors tend to make the natural gut strings very expensive to produce, and price has inhibited widespread use, especially with beginning and intermediate level players.
In the past several years, there has been a tremendous rise in popularity of racquet sports. This has led manufacturers to seek alternatives to natural gut which would be easier and less expensive to produce. Fortunately, the development of synthetic fibers has supported the creation of different synthetic strings. Synthetic strings are made in single-strand coreless construction and in composite integral structures having an internal central core. The central core typically consists of a large diameter single strand or fiber bundle core, wrapped with one or more layers of strands, helically wound and bonded to form a sheath around the core. The wrapped core is then encased in an integral compatible plastic coating.
Such synthetic strings offer the advantages of lower cost of production, good durability, and generally humidity resistance. There are many variations in the way synthetic strings are made as evidenced by the patents ranted in this field. Because of the development of new fibers, bonding agents and manufacturing techniques there is a continuing need for new and improved sports racquet strings made from synthetic materials.
Although sports racquet strings made from synthetic materials constitute the vast majority of today's market, synthetic fiber strings are quite often excessively smooth in their outer surface and are deficient in the extent to which they are able to frictionally engage the ball or other playing object. This results in a dull trajectory and ball control difficulty, especially with maneuvers such as the application of "topspin" or "slice." In order to produce better frictional engagement with the ball and to prevent the ball from "slipping" off the smooth surface of the strings, various countermeasures have been utilized--roughening the outer surface of the string by grinding with abrasives, surface coating the string with frictional or rubbery substances, twisting or braiding the synthetic fiber multifilaments, as well as many other proposals.
Synthetic strings produced in such manner tend to have poor dimensional stability and are reduced in the stiffness and the limit of elasticity to result in tension loss during play. Further, they are inferior in durability because they often exhibit surface aberrations, wearing or breakage due to degradation of the resin, and abrasion, peeling or denaturing of the treating substances. Moreover, since the above-mentioned treatments constitute additional steps, there is an increase in production costs.
A somewhat improved string construction has been suggested which is created by wrapping a nylon monofilament or spaced spiral wrap helically around the outer circumference. Several versions of this type of construction are in use; however, they have not captured widespread appeal since they are difficult to install in racquet frames and tend not to play too well.
Because of the shortcomings inherent in the various attempts to create a synthetic string with more friction, most sports racquets are strung with synthetic strings that have a round profile. Besides the amount of friction a string may have with the ball, there are certain other properties any string should ideally have, depending upon the particular racquet sport, which will affect the string's performance under playing conditions. For instance, there is a particular percentage range or elongation and a particular percentage of resiliency it should have; a minimum amount of tensile strength and knot strength; etc. Within the basic parameters many types and brands of successful strings have been produced, marketed and used in the different racquet sports.
For the sake of explanation, it has been found that in the game of tennis there should be enough elongation so that, upon impact, the ball would be "cupped" in the string bed, that is, in the area within the head of the racquet where the strings are installed in any of several crisscross patterns. This "cupping" of the ball allows the ball to remain on the string bed for a fraction of a second longer and the player is able to impose more control over the direction in which he can return the ball, and he may also impose the effect of "top-spin" (in which the ball leaves the string bed spinning forward, or in the direction of its trajectory), or the effect of "slice" (in which the ball leaves the string bed spinning backward, or in the direction opposite to its trajectory).
These effects, topspin and slice, have become important factors in the games of a great number of tennis players in the intermediate, advanced and professional levels of play.
The way in which topspin is produced is by impacting the ball obliquely in a direction beginning with the racquet frame head below the ball and continuing at an angle carrying the racquet forward and upward. The dynamics of this type of stroke have the effect of producing friction between the string bed and the ball's surface, causing the ball to leave the string bed spinning forward on axis as well as moving forward physically.
The benefit of the topspin effect is that the ball can be hit much higher since, at its apex, the aerodynamics of its rotation will cause it to drop downward more sharply, and upon impact with the court surface, to bounce high and away, much more than a ball with no spin.
An opponent player, stationed at the net in the middle of the court, may easily be able to reach and return a ball hit with no spin; however, the high trajectory of a ball with a good degree of topspin may be too high for him to reach, yet will fall safely within the playing limits of the court.
The method by which slice is produced is by impacting the ball obliquely in a direction beginning with the racquet frame head .above the ball and continuing at an angle carrying the racquet forward and downward. The dynamics of this stroke have the effect of producing friction between the string bed and the ball's surface, causing the ball to leave the string bed spinning in reverse direction to its axis as well as moving forward physically.
The benefit of the slice effect is that the ball tends to drop to the court surface sooner and, upon impact, bounce with less height but with more speed, than a ball hit with no spin.
An opponent player has much more difficulty returning a ball with slice than one with no spin because the sliced ball travels lower, dips to the court surface faster and bounces lower and faster than a ball hit with no spin.
Historically, players have learned the benefits of these effects and many have incorporated one or both into their daily playing techniques. Several years ago, tennis professional Bjorn Borg became successful using large amounts of spin in his play. This was quickly copied by other tennis players with increased enthusiasm, so that an increased awareness of both "topspin" and "slice" swept through the tennis profession.
Tennis companies, wishing to capitalize on this interest, began producing "rough" strings in various ways which, they hoped, would allow the player to enhance whatever amount of topspin or slice he or she could impart on the ball. A typical example, already mentioned, was to provide ordinary round strings with an extra monofilament spiral glued in place around the strings in the hope that the resulting roughness would have the desired effect. There are several limitations to this technique--the extra monofilament spiral made the string difficult to install in the racquet causing widespread dissatisfaction among stringers; the extra monofilament was difficult to glue and often would break free of the base string; and usually this type of string was a simple, low-technology monofilament which lacked the inherent properties necessary for good play.
Players made their own modifications, using sandpaper to roughen the surface of their strings to allow them to "bite" the ball surface more. Stringers, too, became involved. A type of stringing, called "spaghetti stringing," was developed which employed incorporation of foreign materials (knots, etc.) in the string bed. In short, tennis players have been searching for physical changes in the surface of regular round strings by which they can enhance the effects of both topspin and slice.
Although a certain measure of success has been achieved with the foregoing spin performance increasing methods, there remains a demand for a "rough" string which offers the inherent properties for good play, is as easy to install as ordinary round string, and which allows the player to greatly enhance the effects of topspin and slice. It is an object of the present invention to provide such a string.